1. Field of the Invention
The present invention relates to a method for coating a group 4 semiconductor surface composed mainly of a group 4 semiconductor element such as silicon, germanium, carbon, tin and/or lead with a group 4 element or a transition metal element, a process for the production of nanoparticles utilizing said method, and a light emitting element using said nanoparticles, and an optical element such as an optical modulator based on electrooptic effect.
2. Background Art
Research for silicon as an optical material aiming at application to optical information communication fields is historically old and has been carried out for a long period from the middle the 1980's up to the present date. As compared with research for newer optical materials such as indium phosphorus, gallium arsenic, and lithium niobate as an insulator which are group III-V compounds, however, research for silicon has not been advanced.
There are two major reasons for this. The first reason is that silicon does not have any light emission mechanism. That is, silicon does not exhibit luminescence. Secondly silicon does not exhibit any electrooptic effect (EO effect) such as Pockels effect and Kerr effect. These reasons limit the technical application range of silicon, and, thus, it is considered that silicon has not drawn attention as optical materials. EO effect is a generic term for a phenomenon in which the application of an external electric field to a transparent material causes a refractive index change. When the refractive index change is proportional to the intensity of the electric field, this phenomenon is called “Pockels effect,” while, when the refractive index change is proportional to the square of the intensity of the electric field, the phenomenon is called “Kerr effect.”
The reason why silicon does not exhibit luminescence is that silicon is a semiconductor material having an indirect band gap and thus has a crystal structure which cannot produce an efficient light emitting element such as semiconductor lasers. By contrast, indium phosphorus and gallium arsenic as group III-V compounds are semiconductor materials having a direct band gap and thus have often been used as semiconductor laser materials.
The reason why silicon does not have any EO effect is that the crystal structure is highly symmetrical. When a material having EO effect is used, high-speed modulation of laser beams can be realized. More specifically, the preparation of an optical modulator for converting electric signals to light signals can be realized. In fact, an optical modulator has been prepared using lithium niobate having Pockels effect and has been extensively used in the field of optical fiber communication.
Six representative material techniques which have been studied for providing imparting luminescent function to silicon are summarized in Table 1. Specifically, for liquid phase synthesized silicon nanoparticles, rare earth ion doping, gas phase synthesized silicon cluster, porous silicon, silicide semiconductor, and strained superlattice, problems and the like are shown in this table.
TABLE 1Related artinvolvingMainLuminescenceluminescenceLuminescentluminescencequantumStructureof SisitewavelengthyieldcontrollabilityProblem/remarks1Liquid phaseNano-particle 400 nm10%XLuminescence in the regionsynthesizedfrom green to red is hardlynanoparticleseen due to small particle* Grignarddiameters (small particlemethodproblem).2Rare earth ionEr1500 nm10%XEr ions are substantiallydoping (Er)insoluble in Si, making itimpossible to ensureluminescence intensity.3Gas phaseCluster 900 nm 1%XEtching is necessary forsynthesizedobtaining visible luminescenceclusterdue to large particle diameter(large particle problem).4Porous siliconNano-particle? 600 nm 1%XReduction in particle diameter(Si/SiO2)Surface?by etching is again necessaryfor obtaining luminescence inthe region from blue to green.5SilicideFeSi21500 nmUnknownXIndirect semiconductor whensemiconductordetermined based on(β-FeSi2)calculation. Luminous uponstraining.Luminescence intensity isvery weak at roomtemperature.6StrainedSi/SiGe1000 nmUnknownXVisible luminescence is notsuperlatticeobserved. Under(Si/SiGe)development as high-mobilitymaterial rather thanluminescent material.
It is first concluded that all the material techniques have not been led to efficient luminescence of silicon materials. The first technique, the liquid phase synthesized silicon nanoparticles, suffers from a “small particle problem” due to which only nanoparticles with small diameters can be produced. Due to excessively small diameters of the liquid phase synthesized silicon nanoparticles, luminescence takes place mainly from ultraviolet to a blue-violet region, and luminescence in a visible region is difficult.
For the second technique, the rare earth ion doping (Er ion doping), luminescent Er ions are substantially insoluble in the silicon crystal, rendering doping difficult. Consequently, luminescence is disadvantageously difficult.
In contrast to the liquid phase synthesized silicon nanoparticles, the third technique, the gas phase synthesized silicon cluster, suffers from a “large particle problem” in which only particles with large diameters can be produced. The cluster mainly emits light only in an infrared region and does not emit light in a visible region.
As with the gas phase synthesized silicon cluster, the fourth technique, the porous silicon, suffers from a “large particle problem.” In anode oxidation, since the particle diameter is large, luminescence in a red region is possible, but on the other hand, luminescence in a blue or green region is less likely to occur.
The fifth technique, the silicide semiconductor (mainly β-FeSi2), cannot disadvantageously emit light without introduction of stains into the material. Both the results of calculation and the results of experiments show that β-FeSi2 is a semiconductor having an indirect band gap, that is, have revealed that β-FeSi2 is essentially a non-luminescent material.
For the sixth technique, the strained superlattice (Si/SiGe), suffers from a problem that particles having this superlattice hardly emit light. It is being clarified that SiGe is inherently a semiconductor having an indirect band gap, due to which luminescence does not occur without difficulties.
As described above, the conventional material techniques which aim to impart luminescence function to silicon materials could not have actually led to realization of efficient luminescence of silicon materials.
Imparting a luminescent function to silicon can bring about a significant advance of electronics for the following reasons and can be expected to greatly contribute to realization of a wealthy future society.
In the coming future society, it is considered that there would be an ever-increasing social demand for full utilization of information and an ultrahigh speed which is higher than the current speed would be required of electronics including LSIs. Due to this tendency, it is predicted that the signal source in the information processing and the information transmission would be shifted from electrons, the current leader, to light having the maximum information processing and transmission speeds.
In the field of optical information communication, replacement of individual optical devices supporting optical fiber communication from the viewpoint of hardwares. (for example, semiconductor lasers and optical modulators) with silicon would lead to the arrival of time in which LSI and optical fibers are integrated on a silicon wafer, which in turn would lead to harmony of LSI and optical fibers to each other. This means that electronics is integrated with communication and information processing and transmission speeds are increased to the maximum of light speed. Accordingly, siliconization of optical devices can greatly contribute to the realization of ultrahigh speeds in various applications and social infrastructure where broadbands are required, for example, in personal computers, search (the Internet), image recognition systems, communications, and forecasts and predictions (computations and calculations).
As described above, however, the conventional material techniques involve an essential problem that efficient luminescence in silicon is impossible.